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Terrestrial LiDAR (3D Laser Scanning)

Digital Preservation Technologies

by Justin Barton

January 1, 2010

For most endangered heritage sites, the first tool required for preservation and restoration is a reliable, accurate site survey. Terrestrial LiDAR (aka 3D Laser Scanning, aka High Definition Survey) technologies provide 3D survey data that is more accurate and more economically produced than information from surveys using traditional techniques. 3D laser scan data can easily be converted to CAD and other imaging programs for conservation, management, and restoration works as well as virtual tourism, education, and information dissemination.

Laser scanning technologies are the latest development in survey documentation and recording. They employ laser beams that scan a subject, creating a cloud of accurately measured points in a matter of seconds. This raw set of data, known as a "point cloud," contains millions of measurements, accurate to millimeters or fractions of a millimeter, with each point precisely referenced with x,y,z coordinates relative to all other point locations. The point cloud can be viewed immediately upon scanning, providing the immediate visualization of the data as a 3D image. By mapping the points, an accurate 3D model can be created. Traditional methods such as tapes, theodolites and more modern technology such as total stations and GPS provide relatively slow and cumbersome methods for gathering spatial data (Addison 2001). 3D laser scanning can thus be distinguished from traditional survey by the rate at which the physical world is sampled, resulting in high definition data and, correspondingly, very large datasets.

3D laser scanning technologies can be employed to document and record a large variety and scale of objects, structures, buildings, and topographies; literally from a small exquisitely detailed sculpture to a large geo-referenced landscape. Commercially, laser scanning has been found to be very effective in the documentation of structures such as oil and chemical refineries, highways, bridges and other complex structures, producing accurate "as-built" measured drawings in very little time and with very little expense compared to traditional methods. Today, 3D scanning is growing within the Heritage field as an acceptable standard for site documentation. Immediately upon scanning, the point cloud can be used directly to visualize the subject and to accurately access measurements. In addition, the point cloud data can be processed through various software applications to create a variety of deliverables such as CAD document measured drawings, contour maps, 3D models, and animations.

Three dimensional scanning technologies are generally based on one of three methods:
• Time of Flight: A technique by which a laser pulse is emitted from the instrument and the time of flight is measured, from which the distance to the object can be determined.
• Phase Comparison: The instruments emits a stream of light with a known frequency and phase and by comparing the emitted phases to the returned phases the distance to the object can also be determined.
• Triangulation: This system utilizes two sensors which simultaneously record the reflected laser pulse and determines the dim.

For more information on the technologies commonly employed in CyArk's Digital Preservation projects, see the Related Articles below.